Microelectromechanical systems (MEMS) devices are particularly well suited for detection of loading in applications where space is limited or compact size is a necessity. MEMS sensors utilizing piezoresistive sensing schemes are commonly used for pressure and tactile sensing applications, both of which operate on the principle that the applied load creates a stress distribution in a piezoresistive sensor region, and electrical output signals are produced to allow quantification of this load [1-13]. Despite the similarities in the operation of these devices, differences in the geometric design allow devices with varying sensitivity and differing axes of detection to be developed.
Pressure sensors commonly make use of piezoresistors in Wheatstone bridge configurations [2, 4, 8-10]. This type of MEMS sensor detects deformation of a membrane in response to a pressure differential, and as such is only sensitive to 1D input. Piezoresistive membrane devices have also been used to measure interfacial forces [3, 5-7, 11-14]. In these devices, multiple sensing elements can be implemented on each membrane to resolve shear and normal loads in combination. This 3D detection is achieved by examining localized stress distributions that allow differentiation between the load directions. Resolving shear and normal loads with one compact device is of particular interest for tactile applications for robots, force feedback apparatus, as well as for surgical applications [1, 15-21].
A challenge associated with 3D interfacial sensor development is achieving comparable sensitivity in the detection of shear and normal forces. Several groups have reported devices with lower sensitivity in the shear directions [1, 7, 11, 14, 21], while other groups have achieved higher sensitivity in the shear directions [3, 6, 12, 15, 19]. The design used to evaluate shear and normal sensitivities was a square membrane constructed of single-crystal silicon with four piezoresistors doped onto the top surface. This design was chosen for its inherent strength, ease of manufacture, and versatility in terms of varying the sensitivity. The four piezoresistors allow the normal and two shear components of applied load to be resolved independently, providing sufficient information to recreate the original loads applied. This design also provides space to apply a mesa, or block, to the top surface of the membrane to further enhance the shear sensitivity. Other research groups have used mesas in various configurations to enhance the sensitivity of their devices [13, 15, 16].
Four-terminal piezoresistive sensing elements or devices can be chosen to detect membrane stress distributions. Four-terminal devices can provide temperature compensation and can be compact, allowing localized stress detection while maintaining larger electrical connection and contact via sizes as compared to a full Wheatstone bridge [22, 23]. Gridchin et al. determined that the theoretical maximum sensitivity ratio of a four-terminal sensor compared to a full Wheatstone bridge is 0.75, however, the actual sensitivity ratio may be higher if any arms of the bridge are in a region of lower stress, thus giving the four-terminal sensor comparable sensitivity to a Wheatstone bridge [24]. Consistency in modeling and manufacturing of the four-terminal sensing elements allowed the circuit noise to stay consistent between devices, allowing better sensitivity comparisons to be made [25, 26].
The changing of specific geometric parameters of an interfacial force sensor can affect shear and normal sensitivities. The main parameters that can be varied are membrane thickness and mesa height, as well as load application sites and piezoresistor locations on the membrane's surface.
It is, therefore, desirable to provide a MEMS force sensor that overcomes the shortcomings in the prior art, and to improve the shear to normal sensitivity ratio of interfacial devices.